1. Field of the Invention
The present invention relates to a method and apparatus for determining a position, more specifically to a method and apparatus capable of determining a position (particularly, a position coordinate) by receiving a radio wave from a satellite.
2. Description of the Related Art
In the GPS, the position coordinate is normally determined on the world ellipsoid (WGS-84) as a reference by receiving a radio wave from a satellite.
The position determination in the GPS can be classified into the static interferometric position determination and the kinematic interferometric position determination.
The static position determination can be regarded as a method of determining a position, wherein several receivers each having an antenna are securely disposed at different points by tripods or the like, and data of carrier wave phase or the like regarding the geodesic position are collected for 30 minutes to several hours by the receivers to determine the base lines securely assigned to the reference points whose coordinates are known.
On the other hand, the kinematic position determination can be regarded as a method of determining a position, wherein a set of an antenna and a receiver is disposed at a stationary reference point whose coordinate is known, and geodesic positions are determined in a short time by sequentially moving another set of an antenna and a movable receiver. In the kinematic position determination, the time necessary for determination at each point is a few seconds, and therefore it is relatively short, compared with that in the static position determination.
The coordinate of the above-mentioned reference point can be determined as a known coordinate from the base point in the WGS-84 system (World Geodesic System 1984) by the static position determination.
As a method of the kinematic position determination in a further developed type, a real time kinematic position determination (hereinafter referred to as RTK) can be used, in which the position can be determined in real time as a result of position determination.
In the RTK, a fixed station comprises an antenna and a receiver associated therewith, which are disposed at a reference point whose coordinate is known, whereas a mobile station comprises another antenna and another receiver associated therewith which sequentially move to determine the position.
In the RTK, radio waves emanating from a plurality of satellites are simultaneously received in both the fixed and mobile stations, and the coordinate of the geodesic point of the mobile station to be determined relative to the known geodesic point of the fixed station can be directly obtained by correlating the position data obtained in the mobile station with those obtained in the fixed station.
In this case, the radio wave having a specific frequency in the fixed station is generally used to transmit the data of geodesic positions from the fixed station to the mobile station.
Actually, a radio wave transmitter (for instance, frequency 400 MHz; output about 10 mW) in the fixed station is always transmitting the data of geodesic position, and at the same time a radio wave receiver in the mobile station receives the transmitted radio wave, so that the receiver is capable of always monitoring the data of geodesic position.
All of the time data used in the GPS survey are supplied from an atomic clock built in each satellite.
The atomic clock has a time-indicating error of less than 1 second to three hundred thousand years, thereby providing an extreme accuracy in time.
However, the time data received by the receiver includes possible errors resulting from various factors before receiving the data, and therefore it cannot be stated that there is an absolute accuracy in the time thus determined. For this reason, the time correction is conventionally carried out in the receiver, based on the time correction coefficients supplied in a navigation message.
In the GPS position determination, the time data at a moment of the position coordinate data being determined is added thereto in some cases. Since the time data is extremely accurate, as described above, the time data are generally obtained by transforming the time data in the receiver.
As a result, it has been recognized that no time correction from the exterior is required, since information on the time treated in the GPS position determination is extremely accurate.